1. Technical Field of the Invention
This invention relates generally to the field of location of radio frequency transmitters. This invention relates more specifically to the algorithms and processing techniques applied thereto for the location of rogue radio frequency transmitters based upon multiple measures of received signal strength.
2. Prior Art
There exist techniques that can provide an accurate fix on a transmitter's location. Generally, however, these techniques require one or more of the following: a priori knowledge about the transmitter, a finite, calibrated geospatial reference frame, a cooperative transmitter, and/or a large, geographically fixed receiver/network infrastructure.
U.S. Pat. No. 7,006,838 B2 to Diener et al discloses an apparatus and method for determining the location of an unknown wireless radio signal source. The unknown source's signal is received at a plurality of known receiving locations where one of the plurality is used as a reference. The source's signals received at all other receiving locations are compared to the reference. A reference signal is transmitted by a known location whereupon the difference of time of arrival between the known reference signal and the unknown source's signal is computed and the location of the unknown source computed therefrom. This system has the disadvantage, however, in that it is not passively listening to an intruding source, rather it must actively transmit a reference signal having the effect of making its presence known.
U.S. Pat. No. 6,993,592 B2 to Krumm et al discloses a location measurement apparatus and process for determining the location of radio-frequency badges where the badges transmit a message containing their unique identifier to receivers at known locations. The receivers measure signal strength of the transmitting badge if it exceeds a predefined threshold. The receivers then transmit data to a central computer as to the time, identification and received signal strength indication (RSSI) for each badge, which is entered into a table. Locations are computed for those badges whose signal strength exceeds the predefined threshold. This system is an example of an approach where a calibrated geospatial reference frame tracks and locates signal sources. Since the system is calibrated on signal strength to representative locations, the received signal strength at any one of a plurality of fixed position receivers from a source is readily matched up. The utility of this system would be significantly diminished where intruding sources were outside of a calibrated geospatial reference frame because the necessary vectors would be unavailable.
U.S. Pat. No. 6,920,329 B2 to Kennedy, Jr. et al discloses an apparatus and method to find the location of two-way radios amongst a plurality of base stations at known geographic points. The invention employs a time of arrival determination which may also be combined with an angle of arrival determination. With this system, antenna characteristics of the receiving base stations is critical, as well as is their orientation, meaning the system must be well calibrated. This would further suggest that the system has no utility for other than a permanently fixed deployment.
U.S. Pat. No. 6,861,982 B2 to Forstrom et al discloses a method for determining the location of a non-cooperative radio frequency emitter requiring at least three receivers sharing common time with a time reference. Trilateration is employed on signal detection times from emitter to receivers. Since any or all of the receivers may be mobile, communication with the reference provides an accurate adjustment of propagation time in compensation for the receiver's mobility. Since trilateration is employed, this system will have no utility with fewer than three receivers deployed, operating, and in reception of the signal source.
U.S. Pat. No. 7,020,475 B2 to Bahl et al discloses a method for a mobile computer user to determine his location within a building. Wireless base stations are employed at known locations throughout a building. Signal strength is measured and looked up in a table of known locations of base stations and their respective signal strengths. The current location of the mobile computer user is determined as the one corresponding to the most likely base station. The method may be used in the opposite sense where the base stations detect the signal strength of the mobile computer user, provided a reference signal strength versus distance of the mobile computer from the base stations is known. This system, however, depends entirely upon charted signal strengths at predetermined locations within a fixed structure. It would have little utility for geolocating a wider ranging intruder signal source of unknown signal strength.
U.S. Pat. No. 7,019,694 B2 to Krumm et al discloses a method for locating radio frequency transmitters. A plurality of receivers measure and forward signal strength to a central computer. Multiple measured signal strengths attributed to the same transmitter form a locating signal strength vector. Exemplary vectors are generated in a calibration procedure to various locations. The locating signal strength vector is compared to the previously obtained exemplary vectors to determine with which exemplary vector the locating signal strength vector corresponds, so as to determine the location of the radio frequency transmitter. The benefits of this system include reception by as few as only one receiver and compatibility with an existing computer network. The drawback of this system, however, is that it is possible to locate a signal source only if receivers are already placed in, and the network to which they are connected is extended to, every conceivable location a signal source of interest might be.
Angle of Arrival (AoA) and Time Difference of Arrival (TDOA) methods are desirable to use, but they require directional antennas that must be precisely sited and/or complex receiver hardware with additional channel capacity for distributing precise time references. Aside from the time it would take to integrate, calibrate and maintain these components, their addition greatly increases the cost of a wireless intrusion detection system (WIDS).
What is lacking from the prior art is a system that instead employs upon simple omni-directional detection of rogue transmitters without the need for vast databases of a priori information such as calibrated RSSI look-up tables, that, when simply placed in any geospatial location, will immediately begin to detect and geolocate intruding signal sources in a wireless network.
Physical Challenges to Wireless Intrusion Detection
a. Detection of Received Signal Strength
The present invention performs location solely on the non-directional Received Signal Strength Indication (RSSI) provided by the commercial off-the-shelf (COTS) wireless cards. When the present invention achieves a “sighting” of a rogue transmitter, it logs and reports the observation time, RSSI, and other information. The RSSI alone is not a highly useful measurement in that it provides only two facts: an indication of where the measured RSSI falls within the measurement scale relative to the maximum RSSI; and that a stronger intrusion signal has a higher RSSI than a weaker signal.
b. Omnidirectional Radiation Patterns
Most wireless access points utilize omnidirection antennas in their wireless network infrastructure. These will provide the rogue transmitter a source of entry. RSSI in combination with the analytical tools does provide a valid geolocation method for the case of a rogue transmitter. However, it is clearly invalid for the case of a rogue transmitter employing a high directionality, high gain antenna.
c. Two-Dimensional Geometry
If it is assumed that a rogue transmitter and the WIDS receivers are coplanar, i.e., they are all at the same height or altitude, the mathematical analysis can be limited to two dimensions. However, while a two-dimensional analysis is suitable for a drive-by network intruder scenario, for example, it is clearly inadequate for three-dimensional structures such as multi-floor office buildings.
d. Non-Flat Earth
For distances that are less than a few tens of kilometers it is essentially immaterial that the equations used in the analysis of RSSI are based on plane trigonometry rather than spherical trigonometry. Thus, if a grid system is used, it may be assumed only to be based on rectangular coordinates, where the origin, orientation, and scale are arbitrary. For the characteristically small RSSI reception distances it is sufficient to consider latitude/longitude (lat/long) coordinates to be rectangular on a plane, assuming that longitude is scaled appropriately for the given latitude.
e. Radio Frequency (RF) Propagation Effects
Little about RF propagation, particularly through unknown numbers and composition of walls, can be assumed. The single greatest variation seems to be the nearly random factor of radio frequency (RF) attenuation due to the environment: walls, floors, ceilings, people, and other obstructions. In particular, the attenuation due to walls is most unpredictable, as their composition and style of construction vary widely within even a small area of a building. Signal paths that may be clear line-of-sight or travel through one, two, three, or more walls must be accounted for. However, experiments have indicated that, in practice, the ceteris paribus principle held and RF attenuation seemed to balance out on the multiple paths.
f. Necessary Conditions
Rogue network intruders must transmit There is no way for a WIDS receiver to detect a purely passive monitor. WIDS sightings must be simultaneous to detect a mobile rogue transmitter. Stationary transmitters, which transmit periodically, can be located by integrating measurements obtained over a period of time.